Clutch vs. Electric Fan

Why not run an electric fan and keep a mechanical fan under the spare tire or something?? (in case of malfunction)..

I don't have much experience with electric fans so I'm going to ask this...If you are running an electric fan only by a switch, I assume that you would keep an eye on the temp gauge and turn it on when needed, correct? It seems to me that if the electric fan was running constantly-- it would not last very long.